A problem prevalent in image scanning or digitizing systems is the requirement for a calibration operation in order to correct for non-uniformities therein prior to use. Generally, in beam scanning systems and plural element scanning systems, such as charge-coupled devices (CCDs), the sensor(s) must be calibrated. Calibration of a sensor offset is directed to determining the level of the signal in response to reflective or non-reflective regions of the document, for example a black region in a black-and-white document. Calibration also is directed to characterizing the gain of the sensor over a range of reflectances so as to adequately adjust any amplification of the signal to maximize the dynamic range thereof.
In systems employing plural element scanning devices, such as charge-coupled devices, for viewing by raster scanning an original, the output signal produced by the CCD includes a potential attributable to the inherent operating characteristics of the CCD. To restore the image output signal of the CCD to a true or absolute value, the potential derived from the CCD, referred to as the offset potential or signal, must be removed from the image signal. However, if the offset signal that is removed is greater or less than the actual offset signal, a noticeable aberration or distortion in the image output signal may result Since the operating characteristics of a CCD often vary widely from one CCD to another and even vary from time to time for the same CCD or for different integration rates, the accurate determination of the offset signal to be removed is often difficult. The problem is further complicated in systems where multiple CCDs are employed. Moreover, the responses of CCD sensors have commonly assumed to be linear so as to enable the characterization of the response as a first-order function.
To address the offset characterization problem, sensor offset calibration is often accomplished using one of two methods. In a first method, the offset level is determined by measuring the output signal level of the sensor when no exposure light is provided. Unfortunately, this no-light method does not account for stray light that may be present within the exposure cavity and can result in significant errors during offset calibration of the sensor. A second method employs a black calibration target that is exposed using the exposure light source used during normal scanning. This method is employed, for example, by the Xerox.RTM. DocuTech.RTM. Production Publisher scanner. When the sensor is exposed to light reflected from the black calibration target the offset level is determined by measuring the output signal level of the sensor.
Gain, on the other hand, is commonly characterized by measuring a second output signal level of the sensor when exposure light is provided and reflected off a white calibration target, having a known reflectance, and then comparing the first and second output signals relative to the known difference between the reflectance of the black and white calibration targets. This general method is employed, for example, in the Xerox.RTM. 7650 Prolmager.RTM.. Unfortunately, characterization of sensors exhibiting non-linear responses is generally avoided by specifying CCDs with highly linear responses.
Heretofore, various apparatus and methods have been developed to address the sensor characterization problem, some of which are described in the following disclosures which may be relevant:
U.S. Pat. No. 3,586,772 to Hardin, issued Jun. 22, 1971, discloses an optical character reader which employs a clipping level determined as a function of black and white peaks detected during a normalization scan.
U.S. Pat. No. 3,673,322 to Baxter, issued Jun. 27, 1972, teaches a facsimile transmission system having an optical sensor for generating electrical signals in response to the amount of light directed onto it. A rotatable drum used to hold the document includes a highly reflective metal strip which guarantees a maximum white signal is produced once during each revolution. The signal output for transmission varies from a maximum positive level for white to a maximum negative level for black and is determined by the document region being scanned.
U.S. Pat. No. 3,800,078 to Cochran et al., issued Mar. 26, 1974, teaches a digitally compensated scanning system wherein a clear band of background, to compensate for photodiode sensitivity, is assured by providing a suitable margin along the top of a document being scanned. A sensitivity and illumination variance signal is generated and converted for each photodiode in the array, stored in memory as a digitized value, and then used to provide corrected video signals.
U.S. Pat. No. 3,800,079 to McNeil et al., issued Mar. 26, 1974, discloses a scanning system where photodiode sensitivity and illumination variance signals are removed to provide a corrected video information signal.
U.S. Pat. No. 3,952,144 to Kolker, issued Apr. 20, 1976, teaches a device which performs calibration once every predetermined number of scans. Kolker discloses that a facsimile transmitter makes a preliminary calibrating scan in which the transmitter sequentially scans a known black area and a known white area. An automatic background and contrast control unit stores a first sample of the uncorrected video signal which represents the scanned black area and stores a second sample of the uncorrected video signal which represents the scanned white area. During subsequent scanning, the automatic background and contrast control unit continually produces voltages representing the stored black and white samples and uses these voltages to correct the video signal received during the scanning of the document.
U.S. Pat. No. 4,216,503 to Wiggins, issued Aug. 5, 1980, teaches a device which proposes to correct gain and offset drift due to changes in the operating characteristics of a CCD. The patent discloses a system where dark and light level signals are isolated and processed by a microprocessor unit in accordance with a preestablished routine to provide an offset potential and gain multiplicand. The determined offset potential and gain multiplicand are used to remove the offset and set a signal gain for the next succeeding line of image signals. The process is then repeated for each line of image signals to be output from the CCD.
U.S. Pat. No. 4,555,732 to Tuhro, issued Nov. 26, 1985, is another example of a device that corrects for offset and gain drift. Tuhro discloses an image sensor correction system which maintains the offset voltages in the shift registers of a multi-channel image sensor substantially equal. U.S. Pat. No. 4,555,732 discloses that a pair of control gates permits sampling the current offset voltages in the shift register of each channel to provide an adjusted potential for balancing any differences between the shift registers. Specifically described is a device that compares the various offsets of a plurality of shift registers and determines a single offset potential to be applied to each shift register according to the comparison.
U.S. Pat. No. 5,282,024 to Takei, issued Jan. 25, 1994, teaches a white balance correction device for use in an image sensing apparatus such as a video camera. The device includes a gain control circuit that controls the gain of a color signal output from an image sensor. A similar white balance adjusting circuit is disclosed in U.S. Pat. No. 5,283,632 to Suzuki et al., issued Feb. 1, 1994.
P. Swart teaches in the IBM Technical Disclosure Bulletin, Vol. 14, No. 3, August 1971, a "Contrast Amplifier" that operates based upon a peak white voltage from the document background and a peak black voltage determined when the scanner is in an off mode.
I. Qureshi discloses, in the Xerox Disclosure Journal, Vol. 18, No. 1, January/February 1993 (pp. 75-76) an "Automatic Gain Correction (AGC) Video Correction" that compensates for component variations in the raster input scanner.
These various methods may be unable to reliably and precisely correct for offset characteristics and non-linear sensor responses. Moreover, with the recent development of full-width array systems, the drift changes in the fast scan direction become more prevalent, notwithstanding the system being used; i.e., platen scan or constant velocity transport. This is due to the fact that the full width arrays are typically made of several smaller arrays joined together in a butted or staggered manner. The need for a more precise offset correction method has led to the method of offset correction and higher order response characterization that is the subject matter of the present invention.
In accordance with the present invention, there is provided a method for determining an offset correction level for a light sensitive sensor used to record the intensity of exposure light reflected from the surface of a document, comprising the steps of: measuring the sensor response when the sensor is exposed to light reflected from a first target having a first reflectance level; measuring the sensor response when the sensor is exposed to light reflected from a second target, the second target having a second reflectance level greater than the first reflectance level; and calculating, as a function of the sensor response to the first target and the sensor response to the second target and the reflectances of the first and second targets, the response of the sensor to light reflected from a zero reflectance target, and thereby enabling the calculated response to be used as the offset correction level for the sensor.
In accordance with another aspect of the present invention, there is provided a method for determining a gain correction factor and at least one higher order correction factor for a light sensitive sensor used to record the intensity of exposure light reflected from the surface of a document, comprising the steps of: measuring the sensor response when the sensor is exposed to light reflected from a first target having a first reflectance; measuring the sensor response when the sensor is exposed to light reflected from a second target, the second target having a second reflectance greater than the first reflectance; measuring the sensor response when the sensor is exposed to light reflected from a third target, the third target having a third reflectance greater than the first reflectance and less than the second reflectance; and calculating, as a function of the sensor response to the first target, the second target and the third target and the reflectances of the first, second and third targets, a gain of the sensor, and a higher order correction factor, thereby enabling the calculated response to be used as the gain correction factor for the sensor while further compensating for any non-linear sensor response through the use of the higher order correction factor.
In accordance with yet another aspect of the present invention, there is provided a method for determining an offset correction level and a higher order correction factor for a non-linear light sensitive sensor used to record the intensity of exposure light reflected from the surface of a document, comprising the steps of: measuring the sensor response when the sensor is exposed to light reflected from a first target having a first reflectance level; measuring the sensor response when the sensor is exposed to light reflected from a second target, the second target having a second reflectance level greater than the first reflectance level; measuring the sensor response when the sensor is exposed to light reflected from a third target, the third target having a third reflectance greater than the first reflectance and less than the second reflectance; calculating, as a function of the sensor response to the first target, the second target and the third target and the reflectances of the first, second and third targets, the response of the sensor to light reflected from a zero reflectance target and thereby enabling the calculated response to be used as the offset correction level for the sensor; and characterizing, as a function of the sensor response to the first target, the second target and the third target and the reflectances of the first, second and third targets, a higher order response of the sensor and thereby enabling the characterized response to be used as the higher order correction factor for the non-linear sensor.
In accordance with another aspect of the present invention, there is provided a method for determining offset correction levels for a plurality of light sensitive sensors in a scanning array used to record the intensity of exposure light reflected from the surface of a document, comprising the steps of:
(a) measuring the response for each of the sensors when exposed to light reflected from a first target having a first, non-zero reflectance level; PA1 (b) storing values representing the magnitude of the sensor responses to the first target in a plurality of first calibration memory locations, each first calibration memory location corresponding to a unique sensor; PA1 (c) measuring the response for each of the sensors when exposed to light reflected from a second target, the second target having a second reflectance level greater than the first reflectance level; PA1 (d) storing values representing the magnitude of the sensor response to the second target in a plurality of second calibration memory locations, each second calibration memory location corresponding to a unique sensor; and PA1 (e) calculating, as a function of associated sensor responses stored in the first and second calibration memory locations and the reflectances of the first and second targets, the response of each of the plurality of sensors to light reflected from a zero reflectance target and using the calculated responses as the offset correction levels for each of the sensors.